Trapping [PMo12O40]3− clusters into pre-synthesized ZIF-67 toward MoxCoxC particles confined in uniform carbon polyhedrons for efficient overall water splittingElectronic supplementary information (ESI) available. See DOI: 10.1039/c8sc01454j

Bi-transition metal carbides (BTMCs) are promising in catalytic fields, but the synthesis of small-sized BTMCs remains a challenge. Here, Mo x Co x C (mainly below 20 nm in size) confined in uniform carbon polyhedrons (Mo x Co x C@C) was synthesized based on trapping [PMo 12 O 40 ] 3− (PMo 12 ) clusters into pre-synthesized, uniform ZIF-67 (PMo/ZIF-67). The opening of the windows (0.34 nm) of ZIF-67 cages through heating is essential to allow the trapping of PMo 12 into the cages. This trapping route provides a new method to successfully combine POMs and MOFs that can not be effectively combined via traditional POMOF-based (simultaneous assembly) routes. It also has advantages in controlling the uniformity and components of the materials. The size matching of PMo 12 (1 nm) and the cages (1.16 nm) of ZIF-67 enables effective contact of the Co and Mo sources, thus giving small-sized Mo x Co x C protected by carbon via calcination. The optimized catalysts showed good performance for water splitting with a low η 10 of 83 mV (295 mV) for the hydrogen (oxygen) evolution reaction, which is superior to those derived from ZIF-67 and precursors from POMOF-based routes. Our results also indicated that the HER activity is determined by the kind of BTMC, and the activity for the OER is relative to the oxygen-containing species formed during the initial OER test. A new trapping route has been used to synthesize Mo x Co x C confined in carbon polyhedrons, through trapping PMo 12 clusters into pre-synthesized ZIF-67 polyhedrons. The catalysts showed excellent performance for water splitting.